Titanium dioxide and calcined clay have traditionally been used as filler materials in the making of neutral to weakly acidic paper, to improve the optical properties of the resulting paper, particularly its brightness. These materials, however, especially titanium dioxide, have the disadvantage of being very expensive, which results in a high cost of manufacturing the paper, and consequently, the need to charge a high, uncompetitive price for such paper.
Calcium carbonate, particularly precipitated calcium carbonate, is used as a filler material in the making of alkaline paper. This material results in a paper with enhanced optical properties. Calcium carbonate is also significantly less expensive than titanium dioxide, consequently there are appreciable economic advantages to its use. Calcium carbonate, however, cannot generally be used as a filler in acidic paper because it decomposes in an acidic environment. Consequently, there has long been a need to develop a calcium carbonate based material which is acid stabilized and resistant to decomposition at low pH, so that it can be used as a filler material in the manufacture of acidic paper, such as groundwood paper, where use of an alkaline filler would have a negative impact on the final paper properties.
Heretofore, the use of various phosphoric acids and their salts, especially their sodium and potassium salts, in processes for the production of calcium carbonate by the carbonation of an aqueous suspension of calcium hydroxide, has been known.
Among the literature disclosing such processes are U.S. Pat. No. 4,240,870 wherein at least one of a phosphoric acid such as orthophosphoric acid, metaphosphoric acid, hexametaphosphoric acid, tripolyphosphoric acid, pyrophosphoric acid, tetrapolyphosphoric acid and hexapolyphosphoric acid, or the sodium, potassium or zinc salts thereof is added to an aqueous calcium hydroxide suspension in the first step of a multi-step calcium carbonate production process. The amount of acid or salt utilized is from about 0.01 to about 5.0 weight percent of the calcium hydroxide in the suspension.
Similarly, in U.S. Pat. No. 4,244,933, the reaction of the first step or the second step of a multi-step calcium carbonate synthesis is carried out in the presence of at least one of a phosphoric acid and a water soluble salt thereof. The phosphoric acid or salt thereof is chosen from among the same list, and utilized in the same amount as given in the '870 patent, above.
U.S. Pat. No. 4,018,877 discloses the addition of a complex-forming agent such as a polyphosphate, particularly sodium hexametaphosphate, during the end of the first carbonation stage of an multi-step calcium carbonate production process, preferably after the bulk of the calcium carbonate precipitation has occurred, or during the subsequent ageing or second carbonation step. The amount of complexing agent used ranges from 0.001 to 5 weight percent of the calcium carbonate produced.
U.S. Pat. No. 4,157,379 similarly discloses the addition of a soluble metal salt such as an alkali metal phosphate, after primary carbonation of an aqueous suspension of calcium hydroxide. The amount of salt added is from about 0.001 to 0.5 mole percent of calcium hydroxide in the starting suspension.
Published Japanese patent Application No. 090,821/60 discloses a process for the preparation of calcium carbonate in which a condensed phosphoric acid or its salt is added to a viscous gelatinous emulsion formed by the carbonation of an aqueous calcium hydroxide dispersion with a carbon dioxide-containing gas. The condensed phosphoric acid may be hexametaphosphoric, pyrophosphoric, tripolyphosphoric, polyphosphoric or ultraphosphoric acid.
Published Japanese Patent Application No. 090,822/60 discloses the same basic process as is disclosed in published Japanese Patent Application No. 090,821/60, above, but further including the presence of a magnesium-containing compound in the aqueous calcium hydroxide dispersion.
In none of the foregoing references, however, is it disclosed or suggested that the phosphoric acid or salt thereof added during preparation of the calcium carbonate has the effect of making the resulting calcium carbonate product acid-resistant. Moreover, in all of the above processes, the acid or salt addition is to the calcium hydroxide suspension prior to or during carbonation or to the calcium carbonate precursor just after precipitation, rather than to the final calcium carbonate particles.
U.S. Pat. No. 4,793,985 discloses the addition of from 0.2 to 0.4 weight percent of a dispersing agent such as water soluble salts of polyphosphoric acid or phosphates, particularly, sodium hexametaphosphate, to a slurry of ground calcium carbonate, in order to improve solids distribution within the liquid, as part of a process for producing an ultrafine calcium carbonate with an average particle size of less than 2 microns.
Although the above reference involves the phosphoric acid or polyphosphate addition to a calcium carbonate, the addition is to the calcium carbonate in slurry form and nothing is disclosed or suggested about the acid or phosphate rendering the resulting ultrafine calcium carbonate acid-resistant.
The use of polyphosphoric acid and polyphosphates as dispersants or surfactants in slurries of mineral particles, such as calcium carbonate, for use in waste treatment, is disclosed in U.S. Pat. No. 4,610,801.
U.S. Pat. No. 4,219,590 discloses the treatment of calcium carbonate particles of not more than 20 microns average particle diameter with an acid gas capable of reacting with the calcium carbonate, such as the acid gas obtained by heating phosphoric acid, in order to finely uniformize the calcium carbonate particle size and coat the particle surface with the calcium salt of the acid gas. This reference further discloses that when the acid gas is hydrogen fluoride, sulfur dioxide, phosphoric anhydride or a chloride or fluoride of titanium, aluminum or silica, the resulting calcium carbonate particles demonstrate reduced solubility in acids. The process is based on a solid-gaseous phase contact in a fluidized bed type reactor. In utilizing a gaseous contact process, the patent suggests, however, that there are inherent drawbacks to utilizing a method wherein surface treatment of the calcium carbonate is effected by treating an aqueous suspension of calcium carbonate with a solution or emulsion of the surface treatment agent.
Japanese Patent No. 030,812/82 discloses a method for improving the surface of calcium carbonate particles using an aqueous solution of a condensed phosphate, only, such as a metaphosphate or pyrophosphate, which is added to an aqueous calcium carbonate suspension. The method gives calcium carbonate particles an acid resistance and reduces the pH of the particles by 0.1-5.0.